But that's the thing with Doctor Mario though. Ease of development is not all that gets a character in, and him being briefly considered for Brawl (and we know he was only briefly considered because out of the trio of him, Mewtwo and Roy, it's him who has the least amount of content, only Mewtwo out of the 3 can be considered to be cut for 'time constraints', the other 2 were simply lower priority with Doctor Mario being the first one left behind and Roy a bit after). After all cloned characters make up the majority of cut characters, so that is another point against his favor + the fact that even if he is easy to develop so are other characters and it's hard to imagine why they'd use resources and time creating Doctor Mario over other cloned characters old and new. Again he's part of a very repped franchise and is an alternate version of an already pre-existing character, even among clones there's no reason to bring him back over others.
Except that's not what happened. Sakurai didn't have development time for 7 planned characters, who were more than briefly considered. Having files in the game meant they were outright planned to be in. That makes a massive difference. Only 4 of those never became playable at any point(Dixie Kong, Toon Zelda, Toon Sheik, and whatever Pra_Mai was).
And I just don't think you got my point about Sephiroth. Being 3rd party characters already puts them in a prime position to get cut especially since they aren't Sonic, Mega Man or Pac Man. I am not talking about negotiations here, it's simply an issue of priority that comes with being a 3rd party. And DLC is just not an enticing argument, If we get DLC in the next Smash, why have Cloud and Sephiroth be DLC again? Why not have someone else? A point comes where both fans and Nintendo realize that you pay for the same characters over and over again.
Did you not read about how Cloud was fairly lucky to even be base at the time? He costs a lot to get. And we got barely any other SE content too. Actually less than last time during 4's DLC period. Combining him with Sephiroth makes a big difference because both companies get more bang for their buck. SE actually does prefer DLC, as we've known for ages.
How is Cloud so easy to keep if Sephiroth is not? They're pretty much in the same boat at this point. Cloud was never easy to keep in the first place. Hell, nobody from SE is in an easy position. Also, that's not how it works. Negotiations are extremely important. If Sakurai can't get them as base, he can't get them as base. If it's DLC or nothing, it'll be nothing until DLC becomes greenlit and he tries negotiations for again(which describes the 3rd Party Mii Costumes super well since they couldn't possibly all be negotiated for Ultimate's base at the same time as 4's DLC. Only a tiny few at most, and considering they aren't in base Ultimate, it's far more likely SE etc. said "Nope" at that time. Spirits obviously they were fine with, but we can clearly tell some stuff they definitely want as DLC-only(as does other companies, though how many isn't that clear. Sega most likely too, considering when Bayonetta was negotiated for. Anybody who was negotiated for among the Mii Costumes that are 3rd party from the Cloud/Corrin/Bayonetta packs would've been possible to be in Smash Ultimate's base but... weren't. Seems like Sakurai couldn't get them, which ties into my point. Clearly he can't get everything for base)).
And I do agree that Sora would be more likely for base too(as another user said). He's actually cheaper due to less total content, and is a massive deal on his own, being the number 1 on the ballot worldwide. He only barely got in due to luck with Sakurai wanting him, but had to actually meet with a person randomly just in time. It was actually a contrived coincidence that got him in. Even then, him being DLC again is also very likely, same with Hero.
Neither is Palutena being important to Kid Icarus a reason for her staying because Kid Icarus is a dormant franchise with no foreseeable future ahead. That it even has 3 reps is already pushing it when franchises like Pikmin and Animal Crossing have less. Add the fact that she is not the most unique addition to the roster and all she has is being a well-liked character created by Sakurai. Again, Sakurai will try to not cut her but if there's so much room for veterans, it will be hard to justify bringing Palutena back over other characters that offer more.
...That applies equally to Pit as well. They're both just as important, so she's not in some special position over him anyway. Honestly, all 3 are in a great position to return.
And I did give a reason for the Wii Fit but you kinda didn't get it? That the Wii U was leaving the Wii behind is simply not true. The Wii U era was heavily based on the Wii and heck it leaned specifically on the Wii, everything about the Wii U era (including Smash 4) was a heavy, heavy nod to the Wii. In the Switch era... not at all. There's no comparison on that front. But as I said, it's absolutely not impossible for Wii Fit to return, just that I don't think she'd be that high a priority (not to mention that characters are added to rep an era. That's what the Ice Climbers and Mr. Game & Watch do after all)
While kind of true for Ice Climbers, it was more about characters who originated from the NES among a list and could be fun to use. The gimmick played a very important role for IC's getting in. And that "era" was never removed either, outside of literally being impossible to return them due to technical limitations. That's a key factor about their exclusion. It wasn't "era-based". Mr. Game & Watch isn't a Retro character. He's a surprise character, which is the same thing as Piranha Plant, Duck Hunt, and R.O.B. Their era wasn't the actual point behind it, though it certainly helped make them popular to some(and was an easy way to group some together, respectively, for the amiibo. However, that's taking advantage of their existence, not related to why they were added either. Funnily enough, no retro rep even if you include others, were ever removed for being irrelevant, so any "console rep" doesn't apply to that justification either).
Either way, Wii Fit Trainer was not added cause it was a Wii Era character. So it still has no relevance to her being possibly excluded. There's nothing being "moved on from". She doesn't need to necessarily have a game to continue being in. Not anymore than various characters do. In fact, the general reason for the "no longer getting games" was in specific context to clone-ish characters who were not the only character in their franchise. Which coincidentally doesn't apply to Dr. Mario since he's been kept relevant throughout the years(besides that, he's still Mario, heh). It's why cuts for "the same character, but different" aren't very common in Smash either. That's not much of a good reason to ever cut someone.